A major requirement in achieving a satisfactory communication link between a ground station and a satellite is that the ground station antenna point in the direction of the satellite, i.e. that the maximum of the ground station antenna's beam pattern be aligned along the line of sight between the ground station and the satellite. If the ground station is a mobile platform and/or the satellite orbit is geostationary, high or medium earth orbit then the antenna has to track the satellite in order to continuously point in the direction of the satellite so as to maintain a reasonable quality communication link.
In the following description and claims reference will be made to K.sub.u -band and L-band frequency ranges which are generally accepted to be defined as follows: EQU K.sub.u -band: 10.70-12.75 GHz; L-band: 1.49-1.71 GHz.
Various approaches are known for the architecture of antenna assemblies for mobile and non-mobile communication systems. The most common of these is a two-axis mechanical tracking system. The antenna itself may be a microstrip type or another, such as the NEC (see, e.g., Hiroyuki Inafuku, et al. (1989)) or KVH (KVH Industries, Inc., Middletown, R.I. U.S.A.) systems for, respectively, K.sub.u -band and L-band transmissions.
By another mechanical approach a single-axis mechanical tracking system is used, a typical example being the Nippon Steel's single-layer slotted-waveguide array system for K.sub.u -band transmission (Nippon Steel Corporation, Tokyo, Japan).
By yet another approach a combination of mechanical and electrical tracking is used, such as in the Ball communications system (Ball Telecommunication Products Division, Colorado, U.S.A.).
There are also known non-mechanical antenna assemblies for mobile communication systems. One such non-mechanical antenna described by CAL (CAL, Ottawa, Ontario, Canada) employs phase control on one axis and fixed beams on the other. A two-axis electrically-steered antenna assembly employing conventional phase control schemes has been described by TECOM (TECOM Industries, Inc., Chatsworth, Calif., U.S.A.).
All these known antenna assemblies for mobile communication systems suffer from the common drawback of operating in a single frequency band. Consequently, if one were interested in a mobile communication system operating in two different frequency bands then two of the above-mentioned antennas would have to be used which obviously increases significantly the spatial requirements. If the two-band service is provided through two different satellites, a mechanical pedestal cannot serve the two antennas. Furthermore, the antennas of the first three groups mentioned above suffer from the additional drawback of having mechanical-tracking systems which tend to be cumbersome and slow, limited in their angular coverage, and which are not planar and have to protrude from the surface to which they are applied. Thus, if such an antenna were to be mounted on a mobile platform such as the roof of a land vehicle, it would alter the aerodynamics of such platform.
There are known dual frequency planar antenna arrays in the art (e.g., U.S. Pat. No. 5,043,738 and U.S. Pat. No. 5,262,791). However, none of the known antennas of this type are constructed from two independent planar array antenna units each with its own ground plane and capable of operating independently in two frequency bands, that may be widely space apart (as used in satellite communications) with substantially no interference between the two planar array antenna units.